dt-bindings: cpufreq: qcom-hw: add a 4th frequency domain

On new platforms, a 4th frequency domain is used, document it.

Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Acked-by: Manivannan Sadhasivam <mani@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>